It is desirable to be able to measure the respiration rate of a user. For example, heart rate generally increases upon inhalation and decreases upon exhalation, i.e., some heart rate variation is induced by respiration. Heart Rate Variability (HRV) is the variation of the time intervals between heart beats. An increase in HRV is desirable because it is indicative of a heart rate that is variable and responsive to physiological demands. HRV is greatest when individuals breathe at a frequency that is particular to that individual—their resonance breathing frequency (or “resonance breathing rate”). Respiratory Sinus Arrhythmia (RSA) occurs when Heart Rate Variability (HRV) is in synchrony with respiration, shown when variability on an ECG is shortened during inspiration (“inhalation”) and prolonged during expiration (“exhalation”). Thus, it may be desirable to determine a user's respiration rate and whether that rate is in synchrony with HRV.
Some existing systems and methods for measuring a user's respiration rate rely on a change in the impedance of the user's chest. That change in impedance is caused by two aspects of a user's respiration: a change in the volume of gas in relation to the surrounding tissue; and a change in the electrical path length across the chest that is caused by the expansion of the chest. The impedance increases as the gas volume and path length increase. To measure that change, electrodes may be placed on the user on either side of the chest and modulation signals (excitation signals of an alternating current signal at a known frequency) may be passed between the electrodes. A base voltage signal is created between the electrodes by the impedance of the user's chest to the AC current when the user has completely exhaled. A respiration voltage signal is imposed on the base voltage signal by the increase in impedance caused by the user's respiration. To determine the respiration voltage signal, the resulting combined voltage signal is demodulated. The respiration frequency is determined from the resulting demodulated voltage signal.
FIG. 1 is a prior art circuit diagram for a Texas Instruments ADS1292R from the data sheet for the Texas Instruments ADS1292R, which is a low-power, 2-channel, 24-bit analog-to-digital converter. The datasheet for a Texas Instruments ADS1292R discloses that a feature of the ADS1292R is an integrated respiration impedance measurement. FIG. 1 depicts FIG. 56 from the data sheet for the TI ADS1292R. The pin assignments from the ADS1292R are provided in TABLE 1.
TABLE 1NAME; TERMINAL; FUNCTION; DESCRIPTIONAVDD; 12; Supply; Analog supplyAVSS; 13; Supply; Analog groundCLK; 17; Digital input; Master clock inputCLKSEL; 14; Digital input; Master clock selectCS; 18; Digital input; Chip selectDGND; 24; Supply; Digital groundDIN; 19; Digital input; SPI data inDOUT; 21; Digital output; SPI data outDRDY; 22; Digital output; Data ready; active lowDVDD; 23; Supply; Digital power supplyGPIO1/RCLK1; 26; Digital input/output; General-purpose I/O 1 or resp clock 1 (ADS1292R)GPIO2/RCLK2; 25; Digital input/output; General-purpose I/O 2 or resp clock 2 (ADS1292R)IN1N(1); 3; Analog input; Differential analog negative input 1IN1P(1); 4; Analog input; Differential analog positive input 1IN2N(1); 5; Analog input; Differential analog negative input 2IN2P(1); 6; Analog input; Differential analog positive input 2PGA1N; 1; Analog output; PGA1 inverting outputPGA1P; 2; Analog output; PGA1 noninverting outputPGA2N; 7; Analog output; PGA2 inverting outputPGA2P; 8; Analog output; PGA2 noninverting outputPWDN/RESET; 15; Digital input; Power-down or system reset; active lowRESP_MODN/IN3N(1); 32; Analog input/output; N-side respiration excitation signal for respiration or auxiliary input 3NRESP_MODP/IN3P(1); 31; Analog input/output; P-side respiration excitation signal for respiration or auxiliary input 3PRLDIN/RLDREF; 29; Analog input; Right leg drive input to MUX or RLD amplifier noninverting input; connect to AVDD if not usedRLDINV; 28; Analog input; Right leg drive inverting input; connect to AVDD if not usedRLDOUT; 30; Analog input; Right leg drive outputSCLK; 20; Digital input; SPI clockSTART; 16; Digital input; Start conversionVCAP1; 11; —; Analog bypass capacitorVCAP2; 27; —; Analog bypass capacitorVREFN; 10; Analog input; Negative reference voltage; must be connected to AVSSVREFP; 9; Analog input/output; Positive reference voltage(1)Connect unused analog inputs to AVDD.
According to the data sheet for the TI ADS1292R, the modulation signals are supplied by RESP_MODP and RESP_MODN. Exemplary modulation frequencies are 32 kHz and 64 kHz. Also, according to the data sheet for the TI ADS1292R, if the Right Arm Lead and Left Arm Lead are intended to measure respiration and ECG signals, the two leads are each wired into channel 1 for respiration signals and channel 2 for ECG signals. Accordingly, FIG. 1 depicts the Right Arm Lead wired into IN2N and IN1N and the Left Arm Lead wired into IN1P and IN2P. FIG. 1 further depicts that the Right Arm Lead is also wired into RESP_MODP and that the Left Arm Lead is also wired into RESP_MODP.
However, Applicant determined that the result of the circuit disclosed in the data sheet for the TI ADS1292R was unsatisfactory for measuring respiration with the contacts placed at a user's extremities. Thus, there is a need for a system and method for measuring a user's respiration from extremities.